A conventional motor vehicle may be equipped with a radar system which is used, e.g., for detecting obstacles or vehicles traveling up ahead. Such a radar system must be adapted to the special installation ambiency of a motor vehicle. In this context, besides an appropriate resistance with respect to temperature fluctuations, soiling or acceleration, the size of the radar system also plays a role.
European Patent No. 498 524 describes a motor-vehicle radar system having two separate microwave antenna arrangements for the transmitting path and for the receiving path. Both microwave antenna arrangements include one or a plurality of active feeder elements, and one dielectric lens each for focusing the radar beams. The entire arrangement is accommodated in a two-part housing which, in each case, is terminated (i.e., each housing part is terminated) in the beam direction of the radar system by the two dielectric lenses.
In German Patent No. 44 12 770, a motor-vehicle proximity radar warning system is specified which needs only one microwave-lens antenna arrangement both for the transmitting path and for the receiving path. In this case as well, the radar system is accommodated in a housing which is terminated in the beam direction by the dielectric lens of the microwave antenna arrangement. To achieve a small overall depth, a lens with a short focal distance is used, the lens in addition being designed as a stepped lens in order to reduce the lens thickness.
U.S. Pat. No. 5,455,589 describes a compact microwave and millimeter-wave radar that is likewise provided for use in motor vehicles. To attain a small size, in this radar an antenna arrangement is used in which the ray path of the electromagnetic waves is optically "folded". To that end, an antenna arrangement, composed of a focusing lens and a feeder element, is provided with two reflective layers. A first layer, described as a transreflector, is arranged in front of or behind the antenna lens. It allows electromagnetic waves of a specific polarization direction through, while it reflects electromagnetic waves of the polarization direction at right angles thereto. A second layer, described as a twist reflector, is arranged about the feeder element. It reflects incident electromagnetic waves, during which it turns the waves in their polarization direction by 90.degree.. Due to this arrangement, radar beams are reflected repeatedly between the two layers mentioned, both during transmission and during reception. For this reason, the distance between the feeder elements and the antenna lens can be shortened to an amount which is less than the focal distance of the antenna lens alone.
However, even if U.S. Pat. No. 5,455,589 and German Patent No. 44 12 770 apply themselves in particular to the problem of proposing a compact motor-vehicle radar system, none of the design approaches indicated avoids the disadvantage that the area between the feeder elements and the antenna lens remains empty, and thus largely unused. The size of this area is determined by the necessary focal distance of the antenna lens utilized. In all the design approaches described, this space takes up a large part of the overall space needed by the radar system.
The German Patent No. 195 11 982 describes an arrangement for an imaging radar having a transmitter and a receiver. One exemplary embodiment shows that waves from a waveguide are imaged via a collimator lens through an antenna array onto a second lens. Thus, the antenna array receives the waves to be transmitted and the waves to be received, and is arranged between the waveguide and the second lens. However, the objective of this arrangement is not to achieve a small size, but rather an arrangement in which it is possible to dispense with a local receiving oscillator. It is quite essential in this arrangement that the antenna array and, optionally, components mounted on it as well, are provided for processing the signals to be transmitted and/or to be received. The size of the radar is not reduced by this means.